lewis, a. et al. ultraviolete in the conservator's art. 2011

8/3/2019 Lewis, A. Et Al. Ultraviolete in the Conservator's Art. 2011

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Inside The Conservator's Art

A behind-the-scenes look at conserving Egyptian artifacts at the Phoebe A. Hearst Museum ofAnthropology

http://conservationblog.hearstmuseum.dreamhosters.com/?p=674

{ 2011 01 18 }

The d ark side: ultraviolet-induced visible

fluoresce nce as a conser vation tool in The

Conservators Art

The first step in a conservation treatment or condition assessment is examination. When examining anobject, we try to observe as much as possible about how it was made, how it may have been modified,and how it has deteriorated. In order to get the maximum amount of information about the object, we oftenemploy tools to augment our basic senses. These tools can range from simple magnification devices tosophisticated analytical instruments. One frequently used examination tool is an ultraviolet (UV) lamp,which allows us to witness a phenomenon called fluorescence. UV fluorescence can provide usefulinformation, especially about object composition and the presence and location of old repair materials.

Section of painted limestone lintel from the tomb door of Sen-Nedjem (PAHMA 6-19871). In long-wave UV,old repair materials fluoresce. Photograph courtesy of Tom Fuller.


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Cartonnage foot case PAHMA 5-409 in long-wave UV. A drip of unidentified fluorescent material is markedwith a red arrow.

Ultraviolet refers to a portion of the electromagnetic spectrum, the portion with energy of wavelengthranging between about 100 and 400 nanometers (nm). This radiation is of shorter wavelength and greater


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energy than the radiation visible to the human eye, which lies between approximately 400 and 700 nm.The UV section of the electromagnetic spectrum can be divided into three subcategories: long-wave UVbetween 320 and 400 nm (also known as UV-A), medium-wave UV between 280 and 320 nm (UV-B) andshort-wave UV between 180 and 280 nm (UV-C).

When certain materials are bombarded with UV radiation, the absorbed energy causes electrons totemporarily move to a higher energy state. The excited molecules rapidly lose the excess electronicenergy as electrons return back to the ground, or unexcited, state. As the excited electrons transition backto the ground state, they emit energy, always of longer wavelength than the absorbed energy. The emittedenergy may take the form of heat (vibrational energy) or photons (radiant energy). The emission of aphoton is referred to as fluorescence. When the bombarded materials emit photons in the visible range(between 400 and 700 nm), the human eye can observe fluorescence.

The electromagnetic spectrum.

Some pigments, dyes, natural resins, synthetic adhesives and minerals tend to fluoresce in UV. When UVenergy is directed at such materials in a darkened room, they appear to glow in different colors dependingon the material at hand. Thus UV examination and documentation can be a useful tool for detecting andrecording the presence of certain original or post-manufacture colorants, adhesives and coatings. Forexample, when we examined the painted limestone lintel ofSen-Nedjem, which had been restored in the

late nineteenth century before coming to the museum, we could differentiate modern restoration materialsincluding pigments and gesso based on their UV fluorescence.


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Fragment of painted limestone lintel from the tomb door of Sen-Nedjem (PAHMA 6-19871) in long-waveUV. Photograph courtesy of Tom Fuller.

Fragment of painted limestone lintel from the tomb door of Sen-Nedjem (PAHMA 6-19871) in ambientlight. Photograph courtesy of Tom Fuller.

The yellow and red/pink/peach pigments on fragments of a Greco-Roman cartonnage mask fluoresce inlong-wave UV, leading us to speculate about the nature of the pigments and binding media. The distinctiveUV fluorescence also gave us an additional means of comparison between this mask and several similarlyconstructed and decorated cartonnage objects in the collection, all of which may have been produced bythe same workshop.


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Fragments of cartonnage mask PAHMA 6-20116 in ambient light and long-wave UV.

Because exposure to UV energy can pose a health risk to the eyes, we always wear protective eyewearwhen using a UV lamp!

Allison dons protective eyewear before examining an object with a UV lamp.

Posted by Allison on Tuesday, January 18, 2011, at 5:49 pm. Filed under Cartonnage, Conservationtreatments, Historical background, Stone. Tagged UV fluorescence, UV lamp. Follow any responses to thispost with its comments RSS feed. You can post a comment or trackback from your blog.

lewis, a. et al. ultraviolete in the conservator's art. 2011

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